Tissue becomes ischemic when it is deprived of adequate blood flow. Ischemia causes pain in the area of the affected tissue and, in the case of muscle tissue, can interrupt muscular function. Left untreated, ischemic tissue can become infarcted and permanently non-functioning. Ischemia can be caused by a blockage in the vascular system that prohibits oxygenated blood from reaching the affected tissue area. However, ischemic tissue can be revived to function normally despite the deprivation of oxygenated blood because ischemic tissue can remain in a hibernating state, preserving its viability for some time. Restoring blood flow to the ischemic region serves to revive the ischemic tissue. Although ischemia can occur in various regions of the body, often myocardial tissue of the heart is affected by ischemia. Frequently, the myocardium is deprived of oxygenated blood flow due to coronary artery disease and occlusion of the coronary artery, which normally provides blood to the myocardium. The ischemic tissue causes pain to the individual affected.
Treatment of myocardial ischemia has been addressed by several techniques designed to restore blood supply to the affected region. A conventional approach to treatment of ischemia has been to administer anticoagulants with the objective of increasing blood flow by preventing formation of thrombus in the ischemic region.
Another conventional method of increasing blood flow to ischemic tissue of the myocardium is coronary artery bypass grafting (CABG). One type of CABG involves grafting a venous segment between the aorta and the coronary artery to bypass the occluded portion of the artery. Once blood flow is redirected to the portion of the coronary artery beyond the occlusion, the supply of oxygenated blood is restored to the area of ischemic tissue.
Early researchers, more than thirty years ago, reported promising results for revascularizing the myocardium by piercing the muscle to create multiple channels for blood flow. Sen, P. K. et al., “Transmyocardial Acupuncture—A New Approach to Myocardial Revascularization”, Journal of Thoracic and Cardiovascular Surgery, Vol. 50, No. 2, August 1965, pp. 181–189. Although researchers have reported varying degrees of success with various methods of piercing the myocardium to restore blood flow to the muscle (which has become known generally as transmyocardial revascularization or TMR), many have faced common problems such as closure of the created channels. Various techniques of perforating the muscle tissue to avoid closure have been reported by researchers. These techniques include piercing with a solid sharp tip wire, or coring with a hypodermic tube. Reportedly, many of these methods produced trauma and tearing of the tissue that ultimately led to closure of the channel.
An alternative method of creating channels that potentially avoids the problem of closure involves the use of laser technology. Researchers have reported success in maintaining patent channels in the myocardium by forming the channels with the heat energy of a laser. Mirhoseini, M. et al., “Revascularization of the Heart by Laser”, Journal of Microsurgery, Vol. 2, No. 4, June 1981, pp. 253–260. The laser was said to form channels in the tissue that were clean and made without tearing and trauma, suggesting that scarring does not occur and the channels are less likely to experience the closure that results from healing. U.S. Pat. No. 5,769,843 (Abela et al.) discloses creating laser-made TMR channels utilizing a catheter based system. Abela also discloses a magnetic navigation system to guide the catheter to the desired position within the heart. Aita U.S. Pat. Nos. 5,380,316 and 5,389,096 disclose another approach to a catheter based system for TMR.
Although there has been some published recognition of the desirability of performing TMR in a non-laser catheterization procedure, there does not appear to be evidence that such procedures have been put into practice. U.S. Pat. No. 5,429,144 (Wilk) discloses inserting an expandable implant within a preformed channel created within the myocardium for the purposes of creating blood flow into the tissue from the left ventricle.
Performing TMR by placing stents in the myocardium also is disclosed in U.S. Pat. No. 5,810,836 (Hussein et al.). The Hussein patent discloses several stent embodiments that are delivered through the epicardium of the heart, into the myocardium and positioned to be open to the left ventricle. The stents are intended to maintain an open channel in the myocardium through which blood enters from the ventricle and perfuses into the myocardium.
Angiogenesis, the growth of new blood vessels in tissue, has been the subject of increased study in recent years. Such blood vessel growth to provide new supplies of oxygenated blood to a region of tissue has the potential to remedy a variety of tissue and muscular ailments, particularly ischemia. Primarily, study has focused on perfecting angiogenic factors such as human growth factors produced from genetic engineering techniques. It has been reported that injection of such a growth factor into myocardial tissue initiates angiogenesis at that site, which is exhibited by a new dense capillary network within the tissue. Schumacher et al., “Induction of Neo-Angiogenesis in Ischemic Myocardium by Human Growth Factors”, Circulation, 1998; 97:645–650.